For a Precision Agriculture application I need to make calculations in UTM projection. The data will be stored in Postgis and is made of soil samples results (point data) and field boundaries of many farms. But the farms are scattered through many utm zones (ex. a farm is in Z23 and the other is in Z22, so I can't store the data in UTM, it needs to be stored in a SRS without zones.
Since the calculations will be made in UTM the data will always need to be converted back and forth to the storage SRS.
So which of those inputs is cheaper in terms of computations of SRS conversion when the output SRS is UTM Wgs84:
-Geometry vs. Geography Postgis types
-Geometry projected (ex World Mercator) vs. Unprojected (latlon)
I think that it involves the internal Proj4 math but I couldn't understand it myself...